Conventionally, in wireless communication equipment or the like, an amplification scheme has been proposed that uses an EER (Envelope Elimination and Restoration) modulation amplifier having high-efficient modulation and amplification performance. This EER modulation amplifier amplifies a high frequency signal using a class B or class E saturated amplifier, and further carries out amplitude modulation by inputting the amplitude signal to a power supply terminal of an amplification section in the last of the saturated amplifier. Therefore, this EER modulation amplifier has a complex function where a modulation function is added to a linear amplification function of the original high frequency amplifier (for example, see Patent Document 1). In this type of the EER high frequency amplifier, a high frequency amplification section amplifies only the phase signal which does not include amplitude information, and, for example, the high frequency amplification section configured with gallium arsenide FET (GaAs FET) carries out amplitude modulation by controlling the drain supply voltage according to the amplitude signal based on the change in gain of GaAs FET according to the drain voltage. According to this configuration, the high frequency amplification section does not amplify the amplitude signal, so that it is possible to use a high-efficient saturated amplifier for the high frequency amplification section. As a result, it is possible to improve efficiency of the EER high frequency amplifier.
FIG. 1 is a schematic configuration diagram of a conventionally used EER high frequency amplifier. In FIG. 1, when a high frequency signal is inputted to EER high frequency amplifier 10, the high frequency signal is broken down into polar components of the amplitude component and the phase component through envelope detection section 11 and limiter 12. Then, the broken down amplitude component and phase component flow through different paths (that is, amplitude signal path 13 and phase signal path 14), and are separately amplified. That is, when the phase signal flowing through phase signal path 14 is amplified at high frequency amplification section 16, and the amplitude signal flowing through amplitude signal path 13 is amplified at baseband amplification section 15, the amplitude signal amplified at baseband amplification section 15 becomes a power supply voltage of high frequency amplification section 16, and thereby high frequency amplification section 16 carries out amplitude modulation, recombines the amplitude component and the phase component, and outputs a linearly-amplified high frequency signal.
However, with an actual EER high frequency amplifier, unless a width of the dynamic range of gain fluctuation of high frequency amplification section 16 due to a change of the power supply voltage is ensured wide enough for the amplitude component, it is not possible to accurately indicate the amplitude signal which should be originally generated, and, as a result, a desired high frequency signal cannot be outputted. Therefore, high frequency amplification section 16 must realize a wide dynamic range. In particular, when the power supply voltage is made lower, and gain of high frequency amplification section 16 is made lower, leak of an input signal to the output side due to insufficiency of the isolation between the input and the output of high frequency amplification section 16 becomes prominent, and therefore the dynamic range of gain with respect to the power supply voltage may be restricted.
As a measure to improve insufficiency of the isolation, a method is known where a metal plate is inserted between the input and the output of the high frequency amplifier to prevent electrical coupling between the input and the output because of space. FIG. 2 is a conceptual diagram for improving the isolation between the input and the output in the conventional high frequency amplifier. As shown in FIG. 2, metal shield plate 20 is provided between the input and the output of high frequency amplifier 17, and input signal line 18 and output signal line 19 are isolated from the high frequency signal. That is, it is possible to improve the isolation by suppressing spatial electrical coupling between the input and the output of high frequency amplifier 17 using metal shield plate 20 (for example, see Non-Patent Document 1).
Further, a method is known for improving the isolation by providing an impedance matching circuit at the input or interstage of the high frequency amplifier, and providing a voltage variable element in this impedance matching circuit. According to this technique, it is possible to change a ratio between a reflected wave and a traveling wave (that is, SWR) by changing a constant of the voltage variable element of the impedance matching circuit using a control voltage, and reduce a leak signal to the output side by suppressing the quasi input signal by substantially increasing the SWR, so that it is possible to improve the isolation (for example, Patent Document 2).    Patent Document 1: Japanese Patent No. 3207153    Patent Document 2: Japanese Patent Application Laid-Open No. HEI8-222973    Non-Patent Document 1: “New Low Frequency/High Frequency Circuit Design Manual” Masaomi Suzuki, CQ Publishing, 1998